The separation of solid bodies, semisolid bodies and/or viscous substances such as oils from liquids is necessary and difficult in many respects. Particularly awkward conditions prevail when the size and/or consistency of the contaminations vary within wide limits. Also when particles of a small size, e.g. dimensions of less than 100 .mu.m are included in, or constitute, the contaminations the problems of achieving an effective filtration are great, for example the maintaining of a high capacity of the filters. According to the technique used hitherto filters comprising fine mesh filter elements or edge filters are used, the mesh sizes or apertures being adapted to the sizes of the contaminants present. It is well known to those versed in the art that such filters have to be frequently cleaned and exchanged, since the filter openings easily become clogged especially when the contaminated liquid includes bodies in a semi-solid state which are easily deformed and stick firmly to the filter medium. Such bodies as a rule occur in all types of waste liquors and may be constituted of organic substances which are in an advanced state of putrefaction and largely shapeless. Also substances such as oils bring about problems of the type referred to here.
In accordance with a technique applied up to now a combination of backwashing and brushing or scraping of the filter surface is used for the cleaning of a filter. Backwashing implies substantial losses, since pure or already purified liquid has to be used for the purpose, as a result of which the amount of purified liquid and hence the capacity of the filter are reduced to a considerable extent. As a consequence thereof the cost of the purification process is increased. Moreover, under certain circumstances, heat has to be supplied in order to make the purification sufficiently effective. Thus, for example, in the case of strongly adhering substances, heated liquid often is used. This naturally gives rise to undesirable additional costs for the heating of the liquid. Finally, on separating, for example, oils it is particularly difficult to clean a filter, especially if the mesh size is small. In certain applications it is not possible to clean the filters, but they have to be replaced when due to clogging, the flow area of the filter has been reduced to a predetermined value.